The present invention relates to spin coating methods and apparatus in which the need to use volatile organic solvents to carry or dissolve the coating material is obviated by the use of liquid carbon dioxide.
The microelectronics industry is dependent upon spin coating for the deposition of uniform thin films on various flat substrates, including silicon wafers for integrated circuits and glass for optoelectronic devices. Spin coating processes typically involve the injection of a liquid coating onto a flat substrate that is rotating at a high rate of speed such as several hundred to several thousand RPM. A spin coater operates by spreading a thin liquid layer (liquid coating solution) on a substrate using the centrifugal force of a spinning substrate. The spreading liquid layer then forms a fairly uniform film after evaporation that covers up and planarizes topological unevenness leaving pin-hole free films. The liquid coating solution is typically comprised of a material that has low volatility such as a polymeric material or an oligomer. These low volatile materials are then dissolved in a volatile liquid solvent or liquid solvent mixture. The volatile liquid solvents are typically volatile organic solvents (VOC), chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), hydrofluorocarbons (HFC), and perfluorocarbons or mixtures thereof such as methyl ethyl ketone (MEK), ethyl acetate, chloroform, toluene, isoamyl acetate, Freon-113, Freon-22, Freon-134a, Freon-227, perfluoromorpholine, etc. The liquid coating solution then thins in a fairly uniform matter across the spinning flat substrate and the excess liquid coating solution falls off the edge of the substrate and is collected. The thin film of liquid coating solution that is retained on the rotating flat substrate continues to evaporate leaving behind a uniform thin film of the compound that has low volatility. The film typically is comprised of a polymeric material (such as a photoresist, interlayer dielectric) but other materials can also be deposited in this way such as low molecular weight compounds (adhesion promoters, antireflective coatings) or sol-gel precursors.
Sometimes a spin coating apparatus is used to remove a substance from a substrate. For example, to develop an exposed photoresist pattern, a liquid solvent is injected onto a substrate that is or will be rotating at a high rate of speed. This liquid solvent can then dissolve the desired substance off the rotating substrate effectively removing the substance off of the rotating substrate.
Spin coating is a solvent intensive process and accounts for a large portion of the use of solvents by the microelectronics industry. Accordingly, there remains a need for spin coating methods and apparatus that reduce or eliminate the use of VOC, CFC, HCFC, HFC, or PFC solvents.
A first aspect of the present invention is a spin coating apparatus for applying a liquid to a substrate. The apparatus comprises a pressure vessel having an enclosed chamber therein; a turntable for rotating a substrate positioned in the enclosed chamber; a liquid dispenser positioned in the enclosed chamber for depositing a carbon dioxide liquid on that substrate; and a pressure reservoir in fluid communication with the liquid dispenser for storing the carbon dioxide liquid. The apparatus typically includes a pressure controller operatively associated with the chamber that can maintain a pressure within the chamber at about 100 to 10,000 psi.
A second aspect of the present invention is a spin coating method. The method comprises applying a carbon dioxide liquid to surface portion of a substrate; and then rotating the substrate about an axis to distribute the carbon dioxide liquid on the substrate. The carbon dioxide liquid can be distributed on the substrate as a carrier, for the purpose of depositing a material such as a coating on the substrate. In addition, the carbon dioxide liquide can be distributed on the substrate as a solvent, for the purpose of solubilizing, dissolving or removing a material previously deposited on the surface of the substrate.
The foregoing and other objects and aspects of the present invention are explained in greater detail in the drawings herein and the specification set forth below.